This research is concerned with the mechanism of operation of the high affinity iron assimilation apparatus in Escherichia coli and related organisms. The main focus of the work is upon outer membrane receptor for the iron laden form of the ferric specific transport compounds termed siderophores. The receptors for specific siderophores, such as enterobactin, are being purified in order that their chemical and physical properties can be studied. In addition, the location of the siderophore receptors in the outer membrane of enteric bacteria will be ascertained through the use of labelled antibodies. The purified siderophore receptors are being subjected to proteolytic digestion in order to determine the nature of the structure required for biological activity. Binding of alkaline earth cations to the ferric siderophore, ferrichrome and ferric enterobactin, is being studied as a means of enhancing the positive charge and facilitating the permeation of the carriers through the cytoplasmic membrane.